1. Field of the Invention
This invention relates generally to a discharge lamp and, particularly, to the construction of a high output discharge lamp for illuminating an outdoor or indoor studio and the like and having superior cooler rendering property.
2. Description of the Prior Art
For illuminating an outdoor studio and the like, a metal halide discharge lamp has commonly been utilized for its high output and superior color rendering property. Such a metal halide discharge lamp requires the presence of various elements in its discharge space. The light emitted from the lamp consists of the luminous spectrum of these elements. These elements are usually filled in the lamp bulb as bulb as halide and, act such that they dissociate in between the discharge gap, generate the luminous spectrum in receiving the collision energy of the electron. The elements move toward the low temperature zone in the bulb and are formed as the molecule of halide. An amount of heat corresponding to the difference in the electro-negativity (the energy required for coupling) is emitted as heat. The phenomenon is repeated again and again and, is observed as a circulation between the electric discharge zone between the electrodes and the low temperature zone at size tube portions of the bulb. Such circulation can be expressed that the energy generated as luminous spectrum in the discharge zone is converted into the energy in the zone wherein the molecular halide moves toward the low temperature zone.
When the energy conversion is not properly performed, the intensity of the light and the color rendering property are affected. Further, the luminous output decreases due to the dielectric loss, the electrodes will be damaged and the wall of the bulb may be melted and pierced. Further, in discharge lamps other than the metal halide discharge lamp, there is also a tendency for loss in overheating of the electrodes and of the molybdenum foil acting as a sealing member.
The discharge lamp usually comprises rod-like electrodes formed usually of Tungsten rods having confronting ends defining therebetween the discharge gap, molybdenum foils connected respectively with the electrodes and being sealed from the atmosphere, and lead terminals connected respectively with the molybdenum foils and extending outwards of the bulb body. The portions connecting respectively the molybdenum foils with the lead terminals are usually disposed in the atmosphere. Thus, the foils will be contaminated and deteriorated due to the high temperature and be ruptured.
For preventing the deterioration of the molybdenum foils, the length of the sealing portion of prior art molybdenum foils has been elongated so as to increase the heat conductive resistance and to restrict the temperature of the molybdenum foils below the recrystallization temperature. However, in the large output discharge lamp, the electric resistance increases due to the elongated length of the molybdenum foil sealing portion, which induces a rise in temperature and accelerates the deterioration such as by a brittle rupture.